It's been a while since I've done any car alarms, but my old advice was always buy a directed electronics system (they have several brands under their umbrella including viper and clifford). Depending on your vehicle, there might be some additional modules you'll have to buy and usually those are only supported by one brand. Also, always get the glass break sensor. It's usually $20-30 and will detect glass being broken and metal being pried. You can get fake dog shit and like plastic crud like gum and stuff. Stick some of that on the center console and dash and it'll discourage people from wanting to go in your car. Cut some spare wiring harnesses out of a junk yard car and dangle it under the dash as well. Stupid people are afraid of exposed wiring.